1. Field of the Invention
The present invention relates to a process for preparing a self-dispersible coloring agent and an ink composition comprising the coloring agent, more particularly, to a process of preparing a self-dispersible coloring agent by incorporating a hydrophilic group into the coloring agent through a reaction of a hydrophilic group-containing halide and the coloring agent in the presence of a Lewis acid as a catalyst.
2. Description of the Related Art
In general, coloring agents embody their inherent colors by selectively absorbing or reflecting visible light, and are classified as dyestuffs and pigments.
Dyestuffs are dissolved in solvents, and are used in any materials to be dyed, such as fibers, leathers, furs, papers, thus providing considerable fastness to daylight, washing, friction, and the like. Pigments are insoluble in solvents, are coloring agents in the form of particulates, and are not absorbed into the material to be dyed, but are adhered to the surface of the material to be dyed by physical means (e.g., adhesion, and the like), thus providing their inherent colors.
In preparing a pigment-type ink, dispersing a pigment comprising a coloring agent in a solvent is generally carried out. Herein, the size of pigment particles, the distribution of the particle size and the dispersion stability are important factors influencing an efficient dispersion of the pigment. Even if the pigment particles dispersed through the dispersing step described above are stored for a long term or are exposed to the change of environmental factors, such as a change of temperature, aggregation or precipitation of the pigment particles should not occur.
Most pigment-type black inks use a carbon black as a coloring agent. A method for dispersing a carbon black in a solvent for ink using a dispersing agent includes a dispersing method using a styrene-maleic acid anhydride copolymer disclosed in U.S. Pat. No. 3,687,887, or a dispersing method using a hydrophobic polymer dispersing agent containing a hydrophilic group such as a carboxylic acid, a sulfonic acid or a sulfate disclosed in U.S. Pat. No. 4,697,794. A random copolymer that contains a hydrophilic group by incorporating a carboxylic acid is used as a dispersing agent in U.S. Pat. Nos. 5,229,786; 5,172,133; 5,160,370; 5,184,148; or 5,106,417. U.S. Pat. Nos. 5,085,698 and 5,221,334 use an AB-type block copolymer or a BAB-type block copolymer (herein, A is a hydrophilic monomer and B is a hydrophobic monomer) in which a hydrophilic monomer and a hydrophobic monomer are used, as a dispersing agent, U.S. Pat. No. 5,589,522 uses a graft polymer and U.S. Pat. Nos. 4,959,661 and 5,125,968 use a known emulsifying agent.
When using a dispersing agent in order to disperse a carbon black, the dispersing agent is readily available since a general dispersing agent may be used. However, when using a water-soluble dispersing agent to disperse a carbon black pigment in a water-soluble medium, the water-soluble dispersing agent may be adsorbed physically to a surface of the carbon black, and thus, the dispersing effect of the pigment may be obtained only with an excessive amount of the dispersing agent. Thus, since an excessive amount of the water-soluble dispersing agent must be used compared to the amount of the pigment to be dispersed, the overall dispersion efficiency is decreased. Since the dispersing agent does not bind to the pigment efficiently and stably, the dispersion stability becomes ineffective. Thus, when preparing ink using the dispersing agent, long-term storage stability is decreased such that a particle dispersed in the ink is prone to aggregation or precipitation. When a block copolymer and a graft copolymer are used as a dispersing agent, a high cost is required in preparing the dispersing agent. When using a general emulsifying agent, excessive foam occurs during the dispersion process.
To solve the problem incurred when using the dispersing agent and the emulsifying agent, improving dispersion of a carbon black by surface modification of the carbon black was implemented. U.S. Pat. Nos. 5,630,868 and 5,672,198 disclose the method of modifying a carbon black pigment to form a self-dispersible pigment by incorporating a hydrophilic group into the surface of the carbon black through the reaction of the carbon black pigment with a diazonium salt.
However, according to the method, an aromatic amine is required to be in the distal end of a compound that contains a hydrophilic group and reacts with a pigment to transport the hydrophilic group to the pigment. However, such hydrophilic group-containing aromatic amine compounds are difficult to obtain commercially, and the aromatic compound should be reacted with a carbon black after converting the compound to a diazonium salt. Thus, a plurality of reaction processes, not a single reaction process, are required, and the complexity of the process and the cost of the process are increased.
Thus, there is a need for a self-dispersible coloring agent which is easily dispersed in an aqueous liquid medium without using other dispersing agents, which has superior storage stability and dispersion stability, and in which the characteristics of a coloring agent, such as an ability to embody a color, durability, light resistance, and the like are improved, and an ink composition comprising the same.